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1.
Porous fence is a kind of artificial windbreak that has many practical applications. The threshold wind velocities at different distances downwind from porous fences were measured and the corresponding characteristics of particle movement observed to assess their shelter effect. It is found that the fence’s porosity is the key factor that determines the resulting shelter effect. The area near a fence can be typically classified into five regions, each with a different mode of particle movement. Dense fences, and especially solid fences, favor the accumulation of sand upwind of the fences. Fences with porosities of 0.3–0.4 produce the maximum threshold wind velocity; those with porosities of 0.3–0.6 (depending on the fence height) provide the maximum effective shelter distance. It is confirmed that the fence porosities of 0.3–0.4 that have been proposed for practical application in previous research are the most effective for abating wind erosion. 相似文献
2.
地震是极其严重的自然灾害,有可能造成重大伤亡和经济损失。由于城市中人口高度聚集,地震发生时,需将受灾居民快速疏散至应急避难所,因此,应急避难所选址的合理性至关重要。本文提出1种可用于解决选址问题的多准则选址模型,并提出了求解模型的方法。该方法分为3个步骤,首先选择候选应急避难所,其次分析候选避难所的服务范围,最后确定应急避难所的位置。通过案例研究,论证了多准则选址模型在地震应急避难所规划中的有效性。 相似文献
3.
LI Jianhua 《国际泥沙研究》1992,(1)
Field study on wind erosion in the hilly regions with gentle slope in the Northwest Shanxi Province has been carried out from 1983 through 1988. Based on the data observed in the field, quantitative analyses of the wind erosion and the effects of the wind-break forest are presented in this paper in detail. Most of the strong windy days occur in the period of March to May. The coincidence of drought and strong wind in the Spring days with no vegetative cover at all on the loose top soil lead to severe wind erosion in this region. The rate of soil erosion is as high as 12800 t / km2. Major characteristics of wind and other meteorological factors are given in graphs and tables. Wind-break forest reduces the wind velocity by 51. 1 %. Consequently, the amount of wind erosion is reduced by 90% and the dust content in the atmosphere is reduced by 97. 1 % in comparison with those measured in the farmland. This paper provides a preliminary summary on the result of data analyses of wind erosion on the Loess Plateau. It can serve as scientific basis for control of the wind erosion in regions of similar conditions. 相似文献
4.
福建地貌基本特征与农业生产的关系 总被引：7，自引：0，他引：7
本文首先从发展农业生产角度出发,分析了福建地貌的基本特征及其对农业生产的直接和间接影响。继而论述了地貌与农业自然灾害的关系,指出了坡度陡、降水强度大和黑云母花岗岩分布三者相结合的部位是农业生态脆弱的地段。最后,根据福建山河分布格局,提出改善福建农业生态的根本性措施——建立防护林体系。 相似文献
5.
Porous wind barriers are widely used and proven more efficient than solid barriers. In this study, the shelter quality of wind barriers of different porosity, row space and row number was compared based on wind-tunnel measurements. The 14 cm-high (H) scaled models of wind barriers were placed perpendicularly to the wind direction in the wind tunnel, and rough elements were placed upwind of the models to obtain a reasonable self-simulation zone and Reynolds number. The results show that the airflow field changes little in the area above one-row wind barriers. In the area below the models, the airflow field is rearranged into four energy regions. The best shelter effect achieved downwind is always obtained using a 0.35-porosity wind barrier, so we suggest the optimal porosity of 0.3–0.4 for wind barriers in sand-control engineering. The shelter effect comparison of two-row 0.35-porosity wind barriers indicates that 6 H is the optimum row space. Two-row and three-row wind barriers obviously provide better shelter than one-row barriers. Therefore, we suggest 5–7 H as the optimal row spacing for multi-row wind barriers. Our results indicated good agreement with previous studies regarding porosity, row number and row space, with some exceptions about wind barrier design regarding porosity. 相似文献
6.
To investigate the suitability of computational fluid dynamics (CFD) with regard to windbreak aerodynamics, simulations are
performed with a state-of-the-art numerical scheme (Fluent) and compared against experimental data for two- and three-dimensional
disturbances, namely the case of a long straight porous shelter fence and the case of a shelter fence erected in a square
about an enclosed plot. A thorough sensitivity study quantifies the impact of numerical choices on the simulation (e.g. grid-point
density, domain size, turbulence closure), and leads to guidelines that should ensure objective simulation of windbreak flows.
On a fine grid Fluent’s “realizable k–ε closure” gives results that are in qualitative accord with the observed mean winds. 相似文献
7.
林带冬季相立木疏透度及其设计方法的研究 总被引：13，自引：2，他引：11
研究林带冬季相结构设计方法是针对我国北方干旱沙区气候和风沙活动特点,为沙区新垦绿洲防护林体系建设服务的。用随机概率法推导曹新孙立木疏透度与疏透度之间的近似数值关系,得知立木疏透度最佳值为3.3;根据生物器官间生长量的相关性提出林带冬季相结构设计方法,测算了新疆杨、沙枣等六个常用防护林树种的设计参数。 相似文献
8.
Tornado shelter-seeking behavior and tornado shelter options among mobile home residents in the United States 总被引：2，自引：2，他引：0
Residents of 401 mobile homes in Georgia, Mississippi, Illinois, and Oklahoma were surveyed after they heard a tornado warning.
Most residents (69%) did not seek shelter during the warning. Half of those who sought shelter went to the frame house of
a friend, neighbor, or relative, and 25% of those sought shelter in a basement or underground shelter. Some of the places
where residents sought shelter were of dubious quality, such as their own mobile home, another mobile home, or in an out-building.
Twenty-one percent of mobile home residents believed that they had a basement or underground shelter available as shelter
during a tornado warning, and about half of those said they would drive to the shelter. Residents said they would drive if
the shelter was more than 200 m away. Fifteen percent actually had a basement or underground shelter suitable as shelter within
200 m of their mobile home, but only 43% of the residents would use those shelters. The most common reason cited for not using
the shelters was that they did not know the people who lived there. Likewise, a frame house or other sturdy building was within
200 m of 58% of the mobile homes, but only 35% of the residents stated they would use those houses for shelter. Thirty-one
percent of mobile home residents had a ditch that was at least 0.5 m deep within 200 m of the mobile home. However, 44% of
these ditches had utility lines overhead, 23% had water in them, and 20% had trees overhead. The limited tornado shelter options
among mobile home residents in the United States needs to be incorporated into safety instructions so that residents without
nearby shelter are allowed to drive to safer shelter. 相似文献
9.
Wind statistics were measured within a square plot (sidelengthD = 20 m) that was sheltered on all four sides by a porous plasticwindbreak fence (height h=1.25 m, resistance coefficient kr0 = 2.4, porosity p = 0.45), standing on otherwise uniform land (sparse stubble, roughness length z0 0.015 m). At any given point within the plot, short term wind statistics were extremely sensitive to the mean wind direction relative to thefences. Whereas the entire plot was sheltered from a wind blowingnormal to any side of the plot, whenever the wind was oriented soas to blow over a corner, wind reduction was observed onlyover a small fraction of the plot, in the near lee of the upwindfences. 相似文献
10.
近35年三北防护林体系建设工程的防风固沙效应 总被引：3，自引：0，他引：3
通过分析三北工程区生态系统防风固沙服务量变化,进而评价工程的防风固沙效应。结果表明：① 近35 a,三北工程区林地面积持续增加、草地面积持续减少,特别是半干旱风沙区和黄土高原区。植被覆盖度在前20 a持续增加,近15 a则呈现先下降而后略微增加趋势。②土壤风蚀模数持续减少,近15 a减幅远高于前20 a,沙地、草地、其它类型转林地或转草地区域的减幅尤为明显。生态系统防风固沙服务保有率呈上升态势,干旱荒漠区增幅显著。草地和沙地为工程区生态系统防风固沙服务总量贡献了71%,且转草地区域的贡献高于转林地区域。③ 考虑到植被好转同时受气候变化和人类活动的影响,以风场减弱为主的气候变化导致土壤风蚀力减弱与三北防护林等生态工程对生态系统防风固沙服务变化的贡献率分别介于85%~89%和11%~15%之间。 相似文献